Compositions comprising an alkali-soluble resin and a naphthoquinonediazido compound as a light-sensitive material are normally used as positive-type photoresist compositions. Examples of such compositions include novolak-type phenol resin/naphthoquinonediazido-substituted compounds as disclosed in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470. Most typical examples of such compositions include novolak resins made of cresol-formaldehyde/trihydroxybenzophenone-1,2-naphthoquinonediazidosulfon ic ester as disclosed in L. F. Thompson, Introduction to Microlithography, No. 219, pages 112 to 121, published by ACS.
As a binder, novolak resins can be dissolved in alkaline aqueous solutions without swelling. The novolak resin can also exhibit a high resistance, particularly to plasma etching, when an image thus produced is used as a mask for etching. Thus, novolak resins are particularly useful in this application. As a light-sensitive material, a naphthoquinonediazido compound itself serves as a dissolution inhibitor for reducing the alkali solubility of novolak resins but is peculiar in that it undergoes decomposition upon irradiation with light to produce an alkali-soluble substance which rather enhances the alkali solubility of novolak resin. Because of the great change in properties by the irradiation with light, the naphthoquinonediazido compound is particularly useful as the light-sensitive material for a positive-type photoresist.
From such a perspective, many positive-type photoresists comprising novolak resin and naphthoquinonediazido light-sensitive material have heretofore been developed and put to practical use. These positive-type photoresists have attained sufficient results in forming lines having a width as small as 1.5 to 2 .mu.m.
However, integrated circuits have further increased in degree of integration. In the production of semiconductor boards for applications such as SLSI, it has been required to form very fine patterns having a line width of 1 .mu.m or less. In such applications, a high sensitivity photoresists are desired from the viewpoints of particularly high resolution, high precision in producing patterns for exactly copying the shape of the exposure mask, and high productivity.
It is a recent tendency that the etching process is switched from a wet etching process to a dry etching process to improve resolution and enhance the degree of integration in integrated circuits. However, since the dry etching process is subjected to an increase in the temperature of the resist, the resist to be used in this process is required to exhibit a high heat-resistance to avoid heat deforamtion.
Examples of approaches for improving the heat resistance of the resist include the use of a resin free of components with a weight-average molecular weight of 2,000 or less as disclosed in JP-A-60-97347 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and the use of resin wherein the total content of monomers, dimers and trimers is 10% by weight or less as disclosed in JP-A-60-189739.
However, the use of the above-mentioned resin free of or having a reduced amount of low molecular weight components is disadvantageous in that it normally causes a reduction in sensitivity, lowering the throughput in the production of devices.
It has been attempted to improve the sensitivity or developability of a resist composition by incorporating specified compounds into the resist composition. For example, U.S. Pat. Nos. 4,738,915, 4,626,492 and 4,424,270 disclose a positive-type photoresist composition containing trihydroxybenzophenone. The use of such a positive-type photoresist composition containing trihydroxybenzophenone enables improvements in sensitivity and developability but is disadvantageous in that the incorporation of trihydroxybenzophenone causes deterioration in the heat resistance of the composition.
In approaches as disclose in European Patent 301332, JP-A-1-177032, JP-A-1-280748, and U.S. Pat. No. 5,077,173, an aromatic polyhydroxy compound other than trihydroxybenzophenone is used to provide a higher sensitivity without causing any deterioration in the heat resistance. However, these approaches do not result in any improvement in developability.